Over the last 15 to 20 years or so, there has been substantial interest in automotive-type, lead-acid batteries which require, once in service, little, or more desirably, no further maintenance throughout the expected life of the battery. This type of battery is usually termed a "low maintenance" or "maintenance-free battery". The terminology maintenance-free battery will be used herein to include low maintenance batteries as well. This type of battery was first commercially introduced in about 1972 and is currently in widespread use.
A considerable amount of attention over the years has been given to the type of alloys used for manufacturing positive and negative grids in such maintenance-free batteries. When maintenance-free batteries were first commercially introduced, the conventional automotive lead-acid battery normally used grids made from antimony-lead alloys in which the antimony content ranged from about 3-4.5% by weight of the alloy composition. Such alloys were capable of being commercially produced at acceptable rates into battery grids by the gravity casting production techniques then widely used. Moreover, the batteries made using grids of such alloy compositions had desirable deep discharge cycling characteristics.
However, such high antimony content lead-based alloys could not be used in grids in maintenance-free batteries. Thus, the use of such high antimony content alloys resulted in the batteries having undesirable higher gassing, higher self-discharge on stand, and higher attendant water loss characteristics. In other words, batteries with grids made from such alloys accepted high end of charge current during constant voltage overcharge so that excessive gas generation occurred. Accompanying this gas generation was loss of water from the battery electrolyte.
The assignee of the present invention and its predecessors in interest have been in the forefront of research relating to alloys and maintenance-free batteries. Among the patents relating to this subject are the following U.S. Pat. Nos.: 4,006,035; 4,007,056; 4,166,155 and 4,456,579.
Much commercial interest has centered around the use of calcium-tin-lead alloys for use in making grids for maintenance-free batteries. The calcium content in such alloys for positive grids has varied generally from about 0.06 to about 0.1% by weight of the alloy while the tin has generally ranged from about 0.1 up to 0.8% and even more. More typically, the calcium content in such alloys when used for making maintenance-free battery grids has been at least about 0.08% by weight or more.
Other commercial interest for maintenance-free battery grids has been directed to the use of "low antimony" lead-based alloys, viz., alloys containing antimony contents of about 1 to about 2.5%, more typically about 1.5% or so. Use of such low antimony alloys generally required the need to add other alloying ingredients since such low antimony alloys were not capable of being made into grids at acceptable rates under normal production conditions.
Other approaches for grid alloys in maintenance-free batteries have included the use of "hybrid" alloy systems. Most typically, a low antimony, lead-based alloy is used as the alloy for the positive grids while an antimony-free alloy is employed for the negative grids. Often, the alloy of choice for the negative grids has been a calcium-tin-lead alloy or a calcium-aluminum lead alloy.
It has been well recognized over the years that lead-acid batteries are perishable products. Eventually, such batteries in service will fail through one or more of several failure modes. Among these failure modes are failure due to positive grid corrosion and excessive water loss. The thrust of maintenance-free batteries has been to provide a battery that would forestall the failure during service for a period of time considered commensurate with the expected service life of the battery, e.g., three to five years or so.
To achieve this objective, the positive grids used initially for maintenance-free batteries typically had thicknesses of about 60 to about 70 mils or so. The batteries were likewise configured to provide an excess of the electrolyte over that needed to provide the rated capacity of the battery. In that fashion, by filling the electrolyte to a level above that of the top of the battery plates, maintenance-free batteries contained, in effect, a reservoir of electrolyte available to compensate for the water loss, during the service life of the battery. In other words, while the use of appropriate grid alloys will reduce water loss during the service life of the battery, there will always be some water loss in service.
Over the past several years, the manufacture of such automotive lead-acid batteries, typically termed SLI automotive batteries (principally used for the starting, lighting and ignition requirements of an automobile), has become substantially more complex. Battery grids have typically been made by gravity casting (e.g., the molten alloy is fed into what is termed a book mold and is then allowed to solidify, the book mold providing either one or two side-by-side grids). Production equipment using an alternate method to fabricate grids is now commercially available by which battery grids can be continuously formed by expanded metal fabrication techniques. For example, a rolled or wrought alloy strip or a cast strip is slit and expanded using reciprocating dies or the like and then cut into the desired width and height dimensions to form the grid with a lug.
Automobile battery manufacturers thus have available a variety of techniques for forming battery grids in production. However, the effect on performance of the batteries when such techniques are used is not understood all that well. This lack of understanding is particularly evident in view of the factors complicating current SLI battery performance requirements.
One complicating factor in attempting to provide satisfactory service life is the seemingly ever-increasing power and energy requirements demanded in current SLI automotive batteries used in modern automobiles. Many factors have contributed to the need and/or desire for such higher power and energy for such batteries. One major measure of power currently in common usage is the rated number of cold cranking amps. The number of cold cranking amps is considered in the industry as some indication of the relative power of the battery to start an automobile in cold temperature conditions.
Yet another complicating factor is the "under-the-hood" space requirements. Automobile manufacturers have significantly decreased the overall space available for batteries in the engine compartment. Typically, this has required that battery manufacturers provide a lower profile battery, viz., a battery having less overall height than previously required so as to meet current aerodynamic styling needs in automobiles. Such lower profile batteries will have less acid above the plates.
These complicating factors (i.e., a need for increased power and energy with less available space for the battery) have required battery manufacturers to alter the battery internal design configurations to provide the needed power in a lower profile battery container. These internal alterations have typically involved increasing the number of plates used in each cell by employing battery grids with reduced thickness. For example, the number of plates in a BCI Group 24 battery has increased from about 13 to about 19 or so over the last few years while the thickness of the positive grids has decreased from about 65 to 75 mils or so down to about 45 mils and even less in some cases. The reduction in the thickness of the positive grids together with an increase in the number of plates has allowed battery manufacturers to provide Group 24 batteries having rated power output capabilities of 875 cold cranking amps or so. Battery manufacturers currently offer batteries in other BCI sizes having rated power output capabilities of up to 1000 cold cranking amps and even more.
Another aspect that has occurred in recent years is the substantial increase in the under-the-hood temperature to which the battery is exposed in automobile service. Obviously, the under-the-hood temperature is particularly high in the warmer climates. One automobile manufacturer has perceived that, in the past three years or so, the temperature to which an SLI battery is exposed under-the-hood in such warmer climates has risen from about 125.degree. F. to about 165.degree.-190.degree. F. in new automobiles.
The specific temperature increase which is involved is not particularly important. What is important is that such under-the-hood temperatures have in fact increased. The impact of the under-the-hood vehicle service temperature increases on the failure modes has been to substantially increase the occurrence of premature battery failures. The incidence of premature battery failures due to excessive positive grid corrosion has been significant.
One attempt to deal with the acute problem of relatively high under-the-hood temperatures by one battery manufacturer has been to provide a battery designed for such high temperature conditions. This battery goes back to the use of thicker positive grids (about 70 mils or more) while using a smaller number of plates (back down to about 10 per cell). In addition, the head space in each cell is filled with hollow plastic microspheres. The use of such microspheres is perhaps to serve as a vapor barrier to the electrolyte for minimizing evaporative loss of water in the electrolyte or perhaps for limiting heat transfer or the like.
What has not been appreciated in the art is the cumulative effect of all of these complicating factors and increased under-the-hood temperature on the requirements for the battery grid alloy. The overall battery requirements have drastically increased the need for a positive grid alloy that will impart, in the resulting battery, enhanced resistance to positive grid corrosion. As is apparent from the foregoing, a considerable amount of prior work in this field has been directed to calcium-tin-lead alloys for use in maintenance-free battery grids.
Additionally, and more recently, silver-based calcium-tin-lead positive grid alloys have been utilized in sealed, oxygen gas recombinant valve-regulated lead-acid batteries. Such alloys also contain aluminum in an amount of about 0.02 to 0.03% by weight. The calcium content ranges from about 0.09 to about 0.11% by weight while the silver content ranges from about 0.016-0.02% by weight, and the tin content ranges from about 0.5-0.75% by weight.
As previously noted, in addition to forming battery grids by gravity casting, equipment is now commercially available by which battery grids can be continuously cast on a rotary drum grid caster. Additionally, battery grids can also be continuously formed by expanded metal fabrication techniques.
While SLI lead-acid battery manufacturers have available to them this variety of techniques for producing battery grids, some of these techniques have not been successfully commercialized for producing positive grids. The most widely used technique for making SLI battery grids has been the conventional book mold gravity casting technique. It has, however, long been recognized that this technique, semi-continuous at best, can cause several production problems. In the first place, gravity casting techniques are subject to various problems which result in scrap as well as lack of product consistency and the like. These problems include operator errors; wide variation in grid wire thickness and hence overall weight due to mold coating variations and irregularities; substantial material handling in production and difficulty in automating such processes and the accompanying inconsistencies due to human error and the like.
Feeding of these individual grid panels made by gravity casting technique into the pasting machine during high speed production conditions can also result in frequent grid jam ups and with resultant scrap. Further, such jam ups result in production stoppage, lost production, clean-up of jams and variation in paste machine set-up and attendant active material paste weight and thickness variations.
Further, as is known, grids pasted with active material are typically stacked for paste curing prior to assembly of the battery. It is therefore necessary to remove a small quantity of paste surface moisture from the active material paste prior to stacking so that adjacent stacked, pasted plates will not stick together during curing and post-curing, before they are dried. As a practical matter, however, the tendency in commercial production is to surface dry more than is required so as to ensure that any possible sticking problems are eliminated. This further exacerbates the problem of providing product consistency.
Still further, a related problem is the development of what are often termed "checking cracks" or shrinkage cracks in the cured or dried active material paste on the plates, particularly adjacent to the grid wire surface. Such checking cracks can result from either excessive drying or from drying (i.e., moisture removal) too quickly. Such checking cracks not only decrease the expected service life but also the low and high rate discharge performance of batteries using plates having checking cracks because of poor paste adhesion to the underlying grid surface.
Another problem of substantial significance stems from the environmental issues involved in pasting, curing and assembly of batteries using gravity cast SLI battery grids. Lead dust is a major problem, stemming from loss of powdery active material from cured and dry paste during processing and handling while assembling batteries. Mechanical handling loosens powdery active material since there are no surface barriers. The resulting lead dust must be dealt with in an environmentally satisfactory manner, and production staff have to wear respirators while carrying out pasting and battery assembly operations. Indeed, a great many production safeguards need to be provided to handle powdery lead oxide dust.
Potentially, the use of any continuous process like continuous grid casting or other continuous expanded metal fabrication techniques to make battery grids is capable of minimizing, if not eliminating, one or more of the problems associated with gravity casting techniques. There has accordingly been substantial interest and effort directed to the use of such techniques over the years. This effort has resulted in what is believed to be rather widespread use of various continuous, expanded metal fabrication processes for making SLI negative battery grids.
The same benefits would result when using continuous process for making grids and plates for SLI positive battery grids. However, one major issue is present with positive grids and plates that is not an issue with negative battery grids and plates. More particularly, as has been previously discussed herein, corrosion of the positive battery grid is a principal mode of failure of SLI batteries. At least for this reason, as far as can be perceived, expanded metal fabrication techniques have not been widely used commercially for making SLI positive battery grids, because of increased susceptibility of continuous cast strip which is expanded into SLI positive grids to positive grid corrosion. The increasing under-the-hood temperatures discussed herein only serve to exacerbate the difficulties associated with using such expanded metal techniques for producing positive battery grids. Indeed, from the standpoint of customer acceptance, some skepticism has been expressed as to whether continuous expanded metal techniques could be satisfactorily used for commercial production of positive grids and plates.
A principal exception to the foregoing involves a U.S. battery manufacturer who uses a cold-rolled calcium-tin-lead alloy sheet and expanded metal production techniques to make positive and negative battery grids and plates. It is believed that this same general technique has been used for many years. However, what has been occurring at present, it is believed, is that excessive positive grid corrosion is resulting, causing premature battery failure particularly in current automobiles.
In spite of all the considerable work directed to maintenance-free batteries over the past several years, the complicating factors and other aspects previously discussed have created a substantial need for maintenance-free batteries that can meet the power and energy demands required and yet have an adequate service life, particularly when used in warmer climates with elevated under-the-hood vehicle service temperature conditions. The entire automobile service environment and requirements for the battery present an extremely complicated situation which is not all that well understood. A substantial need also exists for a process to continuously produce battery grids that can obviate the problems discussed herein.
The advantages that are provided by sealed lead-acid cells and batteries in comparison to conventional, flooded lead-acid batteries are substantial and varied. Sealed lead-acid battery technology thus offers substantial benefits by eliminating maintenance (e.g., cell watering), environmental (e.g., expensive waste treatment systems and air-borne acid mist) and safety (e.g., acid burns) concerns. Such cells and batteries offer the possibility of operating with virtually no liberation of hydrogen and oxygen during continuous float charge maintenance charging. Well-designed VRLA cells and batteries with good float charge control and thermal balance during operation could result in a 5 to 20 year service life with very minimal maintenance costs.
It is thus not surprising that sealed lead-acid cells and batteries are widely used in commerce today for various applications. Such sealed lead-acid cells and batteries are often term "VRLA" (i.e., valve-regulated, lead-acid) cells and batteries. In stationary battery applications, the sealed lead-acid batteries provide stand-by power in the event of a power failure. For this type of application, such stationary batteries are maintained at a full state-of-charge and in a ready-to-use condition, typically by float maintenance charging at a constant preset voltage. Stationary batteries are used for stand-by or operational power in a wide variety of applications, including, by way of illustration, telecommunications, utilities, for emergency lighting in commercial buildings, as stand-by power for cable television systems, and in uninterruptible power supplies for computer back-up power and the like. Sealed batteries are also increasingly used in "on the road" and "off the road" electric vehicles like fork lift trucks, automated guided vehicles, and pure electric vehicles, whereby these batteries supply all the energy requirements.
There has been considerable effort in this field over the years to develop grid alloys that possess the many and diverse characteristics needed as a positive grid alloy for use in making the positive plates in sealed lead-acid cells and batteries. Indeed, many of the types of lead-based alloys used in conventional flooded lead-acid batteries have been investigated as candidates for use in sealed lead-acid cells. As one example, lead-based alloys including calcium and tin in varying amounts, sometimes with other alloying components, have been used. However, insofar as it is known, none of these alloys have been able to satisfactorily match, much less exceed, the collective favorable properties of the alloys disclosed in U.S. Pat. No. 4,166,155 to Mao et al. and in U.S. Pat. No. 4,401,730 to Szymborski et al. Particular problems which develop when calcium-tin lead-based alloys are used range from the level of mechanical properties that are considered desirable to preventing premature capacity loss (often termed "PCL"). These VRLA cells and batteries work on the fundamental principal of oxygen recombination at the negative active material during charging. The oxygen recombination reaction maintains the negative plate in a partially discharged state and, as a result, the potential is kept near the equilibrium value for the Pb/PbSO.sub.4 system, i.e., .eta..sub.neg =(80 to 100 mV).sup.+, wherein .eta..sub.neg represents the over-voltage of the negative electrode. Since the negative half-cell voltage is polarized by this amount, the positive half-cell electrode will have to operate at a higher voltage which is equal in magnitude to this negative over-voltage shift under a constant or fixed voltage charging. Typically, the cells and batteries used for uninterruptible power systems and telecommunications applications are charged at a fixed constant voltage in the range of 2.21 to 2.35 volts per cell. The consequences of this higher charging voltage on the positive electrode half-cell in continuous float voltage charging in battery service is the higher rate of positive grid corrosion in these VRLA batteries in comparison to flooded lead-acid batteries. Positive grid corrosion rate thus is much higher in the service of a sealed lead-acid battery and could become life-limiting in many instances. The higher rate of positive grid corrosion in VRLA batteries is due to positive half cell voltage shift to higher potentials and also to higher cell temperatures in charging due to the exothermic nature of the oxygen recombination process and lack of an electrolyte reservoir heat sink as is available in flooded lead-acid batteries. Positive grid alloy compositions that have good high temperature corrosion resistance and are free of elemental impurities that could lower either the oxygen or the hydrogen over-voltages would greatly improve the service life of the VRLA batteries in uninterruptible power supply and similar applications.
Substantial interest in calcium-tin-silver lead-based alloys as positive grid alloys has developed. However, the metallurgy of these quaternary alloys is relatively complex due to the inclusion of the three unique solute elements, calcium, tin and silver. This quaternary alloy, in general, is a classic precipitation-hardening alloy deriving high mechanical strength and corrosion resistance attributes due to an uniform dispersion of very fine intermetallic precipitates in a lead rich matrix.
Understanding how such relatively complex metallurgy impacts upon use as positive grid alloys becomes only more complex, and significantly so, when the diverse criteria for positive grid alloys are considered. In the first instance, suitable positive grid alloys must possess acceptable mechanical properties for processability so that the alloy can be made into positive grids of the desired configuration using the desired production method, and be capable of being subjected to various downstream plate processing and assembly steps. In other words, the service life requirements become immaterial if the alloy cannot be made into grids under production conditions and be capable of being processed under desired cell or battery assembling and manufacturing conditions.
The service life requirements for positive grid alloys are diverse and significant. Higher temperature conditions in service heighten the importance of corrosion resistance under such high temperature conditions so as to avoid positive grid corrosion becoming a significant mode of premature failure of performance.
Accordingly, enhancing the microstructure stability of the positive grid in service is an important criteria. Solutions such as making thicker positive grids are not completely satisfactory in some battery designs due to container size limitations, involving economic penalties, while sacrificing capacity and performance.
Moreover, such solutions do not adequately address corrosion issues related to positive grid growth. Positive grid alloys are susceptible to grid growth or dimensional changes due to the corrosion processes encountered in battery service. Excessive positive grid dimensional changes can result in premature failure in service and thus must be carefully controlled. For example, even dimensional changes in either the horizontal or vertical direction as small as 2% to 5% in service could result in complete cell or battery failure.
This grid growth process is the result of application of a small magnitude stress continuously on the grid wire members as the positive paste undergoes phase changes in service. This applied continuous stress results in permanent deformation of the grid wires which leads to stretching of the grid, and, hence, the grid growth. The magnitude of this applied stress increases with increased service temperature, and with decreased cross-sections of the grid wires. Permanent grid wire fractures can occur as this applied stress exceeds the yield point of the alloy. This slow, but steady, state deformation of the grid is sometimes referred to as "creep-induced deformation."
Such creep-induced deformation cannot be completely eliminated. However, minimizing this adverse effect is important, especially for particular types of cells (i.e., VRLA cells) and for grids made using direct cast strip processes.
Other important service life criteria that demand attention are passivation issues. Thus, thermally induced passivation layers in service or storage must be minimized, and hopefully eliminated. Further, passivation effects resulting from discharge conditions must be avoided.
Suitable positive grid alloys must thus develop satisfactorily uniform corrosion layers upon curing of the positive active material to minimize and hopefully eliminate any grid interface-related passivation effects. Avoidance of thermal and electrochemical passivation must be achieved during float charging and open circuit storage conditions.
Further complications arise due to the type of cell or battery as well as the service life requirements. More particularly, the service life requirements for positive grids for conventional flooded electrolyte SLI batteries vary from those for VRLA batteries. Thus, maintaining electrochemical compatibility of the positive grids throughout the demanding oxygen-rich environments in VRLA cells must be accomplished.
Even further, the service life requirements for VRLA cells for motive power applications vary significantly from those for stationary power applications. In the latter, and while creep-induced deformation is an issue which must be addressed in virtually all lead-acid applications, this issue is particularly important in stationary power applications where the expected service life of the batteries is 10 or even 20 years.
Yet another factor that affects these diverse criteria is the manufacturing method selected for making the positive grids. More particularly, the manufacturing method selected can impact upon the alloy composition that is needed to satisfy the desired service life criteria.
There accordingly exists a need for a lead-based alloy which can adequately satisfy the diverse requirements needed for a lead-based alloy for making grids for positive plates used in flooded electrolyte and sealed lead-acid cells and batteries.
It is accordingly an object of the present invention to provide a maintenance-free, lead-acid battery capable of satisfactory service life when operated in relatively high temperature environments.
Another, and more specific, object lies in the provision of an alloy composition useful for making positive grids for such maintenance-free batteries.
A still further and more specific object of this invention is to provide an alloy that can be made into positive grids for such maintenance-free batteries using commercially viable, continuous strip and expanded grid or continuous cast grid manufacturing methods.
Yet another object provides a positive grid alloy for such maintenance-free batteries that will impart enhanced resistance to positive grid corrosion relative to batteries using positive grids made from alloys presently being used.
An additional object of the present invention is to provide an alloy for a positive grid that may be readily formed into a positive grid or a continuous strip followed by grid fabrication using expanded metal techniques or the like without undue loss of any of the key alloying ingredients.
Another object provides a continuous method for making lead-acid battery positive plates characterized by superior high temperature positive grid corrosion resistance.
Yet another object of this invention lies in the provision of lead-acid battery positive plates, and batteries utilizing such plates, characterized by enhanced product consistency relative to the product consistency obtained with gravity cast plates.
A still further object of this invention is to provide a method for making lead-acid battery positive plates that minimizes, or even eliminates, potential environmental concerns such as lead dust and the like.
Yet another object of the present invention is to provide a sealed lead-acid cell or battery utilizing positive grids made of an alloy composition characterized by superior corrosion resistance.
Another object of the present invention is to provide a sealed lead-acid cell or battery utilizing positive grids made of an alloy composition characterized by extended float maintenance charge service in UPS, telecommunications applications, and motive power applications, and where the batteries exhibit very high charge voltage stability due to lack of impurities.
Other objects and advantages of the present invention will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings.